The present invention is a process for the recovery of metals from sulfide ores. More specifically, the process of the present invention relates to the conversion of precious and base metal sulfides to metal chlorides and subsequent recovery of the precious and base metals.
Worldwide there are many ores in which precious and/or base metals are finely intergrown, dispersed or encapsulated within complex sulfide ores. The extraction of metals from these types of ores is either very complicated, expensive, or otherwise not feasible by conventional technology. Thus, incentives exist to develop a process that may economically process such ores.
Precious metal ores that contain platinum group metals (PGMs), gold, and silver are usually associated with sulfide minerals of copper, nickel, and iron. Conventionally, these ores are smelted to separate the sulfide minerals from the gangue. Upon smelting, the sulfide minerals and the precious metals, collect in a separate molten phase, known as matte while the gangue separates as a molten slag. After smelting, the slag is discarded, the matte is cooled, crushed, ground, and leached with sulfuric acid in an autoclave to remove the base metals. The remaining solid residue, often referred to as "leach residue", is further leached with hydrochloric acid and chlorine to recover the precious metals. This process suffers from at least the following major disadvantages: (1) it generates sulfur dioxide which requires expensive gas treating equipment to limit atmospheric emissions; (2) it is long and tedious; and (3) it does not completely dissolve leach residue which must be resmelted.
Other precious metal ores (besides PGMs) that are difficult to treat are gold and silver refractory ores. In refractory gold ores the gold is finely disseminated in a pyritic mineral and cannot be treated by conventional methods, the easiest example of which is direct cyanidation. The gold ore must be treated first. For example, the ore is roasted or leached in an autoclave under oxygen pressure to fully oxidize pyrites. Then, the solid residue from the roasting can be leached with cyanide solutions to recover gold. Depending on the ore and the success of the roasting or autoclaving, gold recovery may vary between 80-95%.
Base metal containing sulfide ore is also processed by smelting. Conventional base metal smelting requires the formation of substantially pure concentrates of individual metals from the metal containing minerals. Complex copper, lead, and zinc bearing pyrite ores produce these pure concentrates at the expense of selectivity and recovery. Economically, it is more desirable to use bulk concentrates which contain a variety of metals because they are less costly to make and can yield higher recoveries. However, conventional metallurgical processes cannot economically treat these bulk concentrates to recover base metals.
Metals within sulfide ores can also be recovered by chlorination. Generally, metals recovery processes that employ chlorination reactions break down into three groups: gaseous chlorination; salt chlorination (in the absence of chlorine gas); or chlorination in a molten salt bath in the presence of chlorine gas. For example, U.S. Pat. Nos. 4,011,146 to Coltrinari et al and 4,362,607 to Ritcey et al teach gaseous chlorination; U.S. Pat. No. 1,883,234 teaches chlorination by salt addition; and U.S. Pat. No. 4,209,501 to Kruesi discloses a molten salt extraction. Furthermore, gaseous chlorination has also been suggested for precious metal recovery, see U.S. Pat. Nos. 4,353,740 to Dunn; 3,825,651 to Heinen et al, and 3,988,415 to Barr.
As explained in H. Parson's "Low Temperature Dry Chlorination of Sulfide Ores --A Review," CIM Bull. Vol. 71, 196 (March 1978), the reaction between chlorine gas and metal sulfides has been known at least since the early part of the century. For example, see U.S. Pat. No. 1,388,086 issued Aug. 16, 1921 to Ashcroft. Many researchers have tried to create commercial processes. They hoped that gaseous chlorination would enable them to: (1) treat complex sulfide ores; (2) produce elemental sulfur; and (3) use less energy.
However, there are some problems inherent in gaseous chlorination processes, e.g., (1) it is difficult to produce separate metal chloride products (2) metal chlorides fuse with the solid residue and cause plugging of the chlorination reactor, (3) sulfur chlorides form, (4) both the metal chloride and the sulfur vapors are difficult to separate and recover, (5) the reaction produces large volumes of gases which cause large losses of concentrate dust, (6) there is poor recovery and recycle of chlorine from iron species, and (7) equipment corrodes.
The second process, chlorination with salt in the absence of chlorine gas, dates back to the 19th Century, for example, see U.S. Pat. No. 589,959 issued Sep. 14, 1897 to Crooke. It also has difficulties. For example, the process uses high temperatures (900.degree.-1000.degree. C.) and requires more energy. Furthermore, at these temperatures the metal chlorides are volatilized and a gas scrubber system is required for their subsequent recovery.
There are some disadvantages to the use of a molten salt bath, the third process. For example: (1) it is difficult to separate a highly pure metal chloride product; (2) in a molten salt process a large amount of salt is required to maintain a fluid slurry; (3) once the ore and the salt bath are reacted, the entire melt must be dissolved to separate the metal chloride product from the solid residue; (4) it is difficult to recover and recycle the chlorides that form the bath; and (5) it is uneconomical to recover chlorine from FeCl.sub.3 that has been dissolved in aqueous solutions. This last difficulty is important because complex ores contain large quantities of iron minerals which consume large amounts of chlorine. Chlorine consumption is economically disfavored.